Abstract
Direct-quenching after hot rolling above recrystallization temperature in austenite was investigated in low carbon and low alloy high tensile strength steels. Effects of cooling rates during direct-quenching on the microstructures, strength, and toughness were investigated, comparison being made with reheat-quenching.
It was clarified that the strength and toughness were remarkably improved by direct-quenching in a certain range of cooling rates, in which the microstructure of direct-quenching showed up as a mixture of martensite and fine bainite, while that of reheat-quenching was coarse bainitic. These results showed that the improvements of the strength and toughness were due to the enhancement of hardenability by direct-quenching.
By calculating ideal critical diameters from tensile strengths of as-quenched steel plates, it became clear that the increase in hardenability by diect-quenching was about by 40%. It was too large to ascribe the increase of hardenability only to the difference of austenite grain sizes in direct-quenching and reheat-quenching.
Some possible mechanisms for these interesting phenomena were discussed.
